State of the art CT body scanners are being modified to produce high resolution images of such traditionally difficult areas as the spine. R. Ethier et al. in 3 Journal of Computer Assisted Tomography 433 (August, 1979) have reported high resolution CT images of lesions in the spinal cord.
Such limited regions of anatomical interest as the spinal cord lie well within the total body portion of a patient positioned within the scanning apparatus. To permit higher resolution in the area of diagnostic significance, the apparatus is made to "focus" on a scan circle smaller than one that would include the entire body section. Accordingly, some portions of the body section will be outside the small scan circle or the "projection" used to reconstruct the image. Acceptable methods of reconstruction require some information from the region external to the small scan circle. There are several techniques for the "completion" of such incomplete or "truncated" projections. See, for example, the copending application, assigned to the assignee of the present invention, Ser. No. 99,067; filed Nov. 29, 1979, for "Projection Completion Method Of Partial Area Scan".
Concentrated high density material, such as bone, situated outside the small scan circle is not considered in previous projection completion methods. Hence, such concentrated masses produce artifacts in the form of streaking in the region of interest. In high resolution spine scanning, this artifact was first noticed when the arms were included in the slice. Having the patient hold the arms overhead eliminated the artifact. However, when the scan included the shoulders the artifact returned. The shoulders, of course, cannot easily be removed from the slice. In addition, in some cases it is not very practical to have the patient hold the arms overhead. This streaking artifact can affect substantially the diagnostic value of images obtained from relatively small regions of interest such as the spine.